TRSHCU ( sheet, ncurvs, curvs, nopts, opts, direct, edges, nedges, which, 
==========================================================================
         original, ifail )
         =================

  Trim sheet body with curves

Receives:
  KI_tag_body              *sheet          --- Sheet body to trim
  KI_int_nitems            *ncurvs         --- number of curves supplied
  KI_tag_curve              curvs[ncurvs]  --- curves to trim sheet
  KI_int_nitems            *nopts          --- no of trimming options supplied
  KI_cod_trsh               opts[nopts]    --- trimming options
  KI_vec_direction          direct         --- direction of projection

Returns:
 <KI_tag_list_edge>        *edges          --- edges imprinted
 <KI_int_nitems>           *nedges         --- number of edges
 <KI_tag_list_curve>       *which          --- which curve edge is derived from
 <KI_tag_list_int>         *original       --- whether edge is original
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_curves_dont_meet       curves not a closed loop
  KI_unsuitable_entity      sheet handed in has no imprintable faces
                            geometry of wrong type
  KI_failed_to_trim         trim would not split the sheet
  KI_fragment               trim would fragment the sheet
  KI_invalid_geometry       geometry fails to pass checks
  KI_bad_selection_code     inappropriate property
  KI_contradictory_request  invalid combination of options
  KI_not_sheet              body is not a sheet

Description:

      This function imprints a list of curves onto a sheet body. It will also
  trim the sheet body if requested. Which part of the sheet body is to be kept
  is controlled by options as described below. The sheet body may have any
  number of faces. The method of projection of the curves is also controlled
  by options. Consecutive curves in a closed loop or open string of curves
  should meet at their ends to modeller tolerance.

      The returned arguments are 3 parallel lists - the first of these is a
  list of the edges imprinted; the second is a list of curve tags which is
  used to indicate which curve each edge is derived from and the third is a
  list of logicals indicating whether each edge is an original edge ( where
  a curve has projected onto an existing edge ) or a new one. KI_true means
  that the edge is original.

      The only method of projection supported at present is where the curves
  are projected onto the body along a given direction given by the 'direct'
  argument. The option for this is TRSHPD. Each point on one of the curves
  should only have one image under projection on the body - i.e. the sheet
  should not be self-obscuring when viewed down the projection direction.

      The option TRSHTR controls whether the function should also perform a
  trim operation on the sheet. If this is not supplied then no trimming is
  done and the operation is similar to a SCRIBE or IMPRINT. If it is supplied
  then a further token must be supplied which indicates which face-set of the
  sheet is to survive the trimming operation:

      TRSHLC - The face-set to the left of the first curve will be retained.
      TRSHRC - The face-set to the right of the first curve will be retained.
      TRSHIL - The face-set inside the loop of curves will be retained
      TRSHOL - The face-set outside the loop of curves will be retained

      These last two options must only be supplied if the curves provided
  constitute a closed, planar loop. If TRSHTR is selected then the imprinted
  edges must split the sheet into at least two face-sets.

      The curves supplied must be of type TYCUPA, TYCUEL, TYCUCI or TYCUTR. If
  they are trimmed curves then the underlying curves mustbe of type TYCUPA,
  TYCUEL, TYCUCI or TYCUST.